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Something New Every Day – Lecture 15 – Structural Forensics

The impact of two fully-loaded 767 aircraft slamming into New York’s World Trade Centre in September 2001 was not what cause the buildings to collapse.

It was the fuel spill.

Forensic engineers determined that the thousands of gallons of aviation fuel that spilled as the two planes exploded consecutively caused fires on six floors of Tower One and nine floors of Tower Two that incinerated everything on those floors over the next hour or so. Contents, occupants, steel supports, concrete floors, everything. Smouldering like a furnace.

As a result, each building effectively had a six- or nine-story ‘void’ within one hour of the impacts and whatever structure did survive was in no condition to withstand the weight of the (respective) 19 and 11 floors above them. The sudden impact of those 19/11 floors hammering down into the void was what caused each tower to pancake and collapse.

Like taking the legs out from under a timber house. The combined weight of the whole thing dropping was way too much for the rest of the building to withstand. Even though they’d technically withstood the impact of the planes.

‘Trails of Evidence: How Forensic Science Works” is a The Great Courses DVD lecture series